Field of the Invention
This invention relates to a deflection yoke for color picture tubes of the type suitable for displaying a color television representation and more particularly to an improvement of such a deflection yoke that requires a less complex dynamic convergence system.
As is well known, in a color picture tube used for color television, three electron beams are produced from an electron gun which is mounted in the neck portion of the tube. To provide a picture, all of the three electron beams must be deflected over the area of the tube screen by the magnetic field of the deflection yoke.
In a shadow mask type color picture tube, for good color picture reproduction it is important that as the beams scan the screen they converge for every point on the color selection shadow mask. Conventionally, the electron gun for a shadow mask type color picture tube is constructed so that the three electron beams converge at the center of the mask or the screen with no deflection field applied. However, when the three beams are deflected over the screen, the point of convergence shifts from the shadow mask surface toward the electron gun and the locus of the point of convergence is generated inside of the mask surface facing the electron gun. Accordingly, for good picture reproduction, it has been customary to employ pre-deflection to achieve the necessary dynamic convergence of the three electron beams. Such dynamic convergence requires the generation of complex parabolic correction signals of proper amplitude and phase relative to the sweep currents in the deflection yoke.
It is also known that the convergence problems associated with such a color picture tube can be simplified somewhat through the use of a deflection yoke producing a non-uniform deflection field. Especially, such a simplification of dynamic convergence can easily be achieved in the case of the plural in-line beams color picture tube in which the three electron beams are arranged in one direction, since dynamic convergence is not required in the direction normal to the plane in which the electron beams lie.
It is also known that for one direction of deflection in an in-line beams color picture tube the meridianal image plane is substantially coincident with the surface of the screen, and for the other direction of deflection the sagittal image plane is substantially coincident with the surface of the screen. To this end, it is common to use a strong pin-cushion shaped horizontal deflection field and a strong barrel shaped vertical deflection field.
However, since the curvature of the sagittal image plane and that of the meridianal image plane are not constants, but functions of deflection distance and of magnetic field distribution, it is impossible to make the focus plane of three beams converging point completely circular. On the other hand, the mask has a single curvature in one direction. Consequently, it is theoretically impossible to determine the magnetic field distribution such that the focus plane of the beams coincides completely with the mask surface. In other words, even with the winding distribution of the deflection yoke improved, it is inevitable that divergence appears between the mask surface and the focus plane of the three beams.